1. FIELD OF THE INVENTION
The present invention relates to a method of producing ferrous sintered alloys which can be used as materials for construction of mechanical elements which require improved wear resistance during high temperature use, such as, for example, valve seats. More specifically, the present invention, provides a method for producing a high density ferrous sintered alloy whose wear resistance is increased by the dispersion of hard particles within the alloy matrix such that the alloy need not be thereafter hardened by a subsequent heat treatment.
2. DESCRIPTION OF THE PRIOR ART
Various process for manufacturing wear resistant ferrous sintered alloys have been heretofore proposed wherein the wear resistance has been increased by the use of high alloy powders, by the employment of hard particles dispersed within the alloy matrix, and by the alloy processing step of heat treatment and forging. Considering the matrix characteristics of ferrous alloys, three techniques may be employed for improving wear resistance: (1) improving the density of the alloy, (2) hardening the alloy matrix, and (3) dispersing hard particles into the alloy. Of course, combinations of these techniques may also be considered.
Forging has been employed to manufacture high strength alloy materials in order to improve the density and harden. However, when utilizing forging, dispersing hard particles into the alloy is not practical in terms of achieving good mold life and the like, and therefore a sintering method has been normally employed to treat the alloys containing the dispersed hard particles so that the density is relatively low.
Furthermore, the prior art method for infiltrating Cu into a ferrous sintered alloy has been to infiltrate the Cu simultaneously with, or at some stage after, sintering. In this prior art method, however, the alloys cannot be mass-produced due to the poor dimensional stability and the poor machinability of the alloy produced, in part because the matrix must be hardened by a rapid cooling step, and in part due to the particular infiltration technique normally employed.
On the other hand, the present invention provides an excellent alloy with respect to machinability due to the fact that a pearlite matrix (which includes ferrite or carbide) is formed into a micro structure after the primary sintering.
In addition, by the application of the present infiltration technique alloy elements and infiltration elements can be so combined and diffused with one another so that part or all of the base structure may be formed into a high hardness martensite or bainite, thus increasing the wear resistance.
Furthermore, in accordance with the present invention, the final cooling rate may be controlled such that the need for additional heat treatment is eliminated.